(1) Field of the Invention
The present invention relates to a direction sensor having an earth magnetism sensor and a rate gyro sensor, and a navigation system of a vehicle having this direction sensor for locating a position on a map. More particularly, this invention relates to an improvement in detection accuracy of a driving direction sensor and navigation system.
(2) Description of the Related Art
Vehicles are recently equipped with a navigation system that detects driving paths, displays the driving position on a map and offers various instructions to aid a driver. The navigation system includes a driving direction sensor and a driving distance sensor, and calculates a position on a map from the driving direction and the driving distance. The direction sensor used in the above system is required to detect the absolute direction corresponding to the direction on a map.
Two types of direction sensors are generally used in the navigation system. One type is a rate gyro sensor that detects a rotation speed and calculates a rotated angle from a reference direction. A conventional gyro having a gimbal, an optical fiber gyro and a gas rate sensor are included in this rate gyro sensor. The rate gyro sensor generally has characteristics such that the detection accuracy is very high when detecting during a short time range, but the accuracy deteriorates when detecting over a long time range due to the accumulation of errors. Therefore, the rate gyro sensor needs to be periodically calibrated. This calibration is performed by or with reference to the absolute direction. Further, the rate gyro sensor needs to be initialized to set the reference direction at a start of the operation because it calculates the rotated angle from the reference direction. This initialization is also performed by or with reference to the absolute direction and the calibration is substantially equal or similar in procedure as the initialization.
The other type of direction sensor is an earth magnetism sensor that detects a forward angle of a vehicle relative to magnetic north of earth and calculates an absolute direction on a map. The earth magnetism sensor has an advantage in that it can detect an absolute direction and detecting errors do not accumulate. However, because the intensity of earth magnetism is as low as 0.3 Gauss, the detection of earth magnetism is disturbed by various external magnetic factors such as polarization of a vehicle body, and a magnetic field produced due to electrical equipment of a vehicle. Further, various facilities such iron bridges, large buildings, high level roads and tunnels also disturb the earth's magnetic field. These disturbances cause errors in the detection of the earth magnetism sensor. Various methods are proposed for compensating the disturbance due to polarization of vehicle body and the disturbance has been reduced by using these methods. However, the disturbances due to facilities such as bridges, buildings, etc. cannot be adequately compensated because these disturbances exist at random.
As the rate gyro sensor and the earth magnetism sensor respectively have the above-mentioned characteristics, a conventional navigation system generally comprises both a rate gyro sensor and an earth magnetic sensor and calculates a precise driving direction by compensating or correcting each other's detection results. For example, in a navigation system disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 62-163721, the errors of earth magnetism due to the polarization of a vehicle is compensated for by the difference of the outputs of the earth magnetism sensor and the rate gyro sensor.
In the most orthodox system, the rate gyro sensor is normally used as a driving direction sensor, and the output of the earth magnetism sensor is used only as a reference direction for the initialization and the periodical calibrations of the rate gyro sensor. This system has problems in that the reference direction obtained from the earth magnetism sensor is not necessarily precise and the accumulation of errors of the rate gyro sensor is not reduced.
In a navigation system disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 62-163721, the errors of an earth magnetism sensor due to the polarization of a vehicle is compensated for by the difference of the outputs of the earth magnetism sensor and the rate gyro sensor.
In a navigation system disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 61-72618, the earth magnetism sensor is normally used as a driving direction sensor and the rate gyro sensor is used when the magnetic field of the location position seems to be disturbed more than a predetermined level. The intensity of the disturbance is determined according to the difference between the outputs of the earth magnetism sensor and the rate gyro sensor in a short time range or interval. However, if the magnetic field of the earth magnetism is uniformly disturbed in a wide range or long time interval, for example, when driving along a railroad or a transmission line, the incorrect output of the earth magnetism sensor is always used because the difference in the short time range is always small under this condition since the disturbance is slowly accumulated.
In another navigation system disclosed in Japanese Unexamined Patent Publication (Kokai) No. 64-353314, information relating to specific facilities that largely influence the earth magnetic field and the influence patterns of the facilities to the magnetic field is disclosed. The earth magnetism sensor detects these changes of the magnetic field due to the specific facilities. The navigation system determines positions of the specific facilities and compensates the locating position according to the positions of the specific facilities. This system normally uses the rate gyro sensor as the driving direction sensor and initializes the rate gyro sensor by the above detected positions. However, the influence pattern of the facility to the magnetic field is very complex. Therefore, it is not easy to determine the facility position according to the detected changes of the magnetic field. Further, when there is no specific facility in a wide range or area, the driving direction is determined only by the rate gyro sensor. Therefore, this system also has a disadvantage in that the error accumulation of the rate gyro sensor increases.